This invention relates to fluid assisted boosters and in particular but not exclusively to vacuum assisted boosters which are typically utilised in motor vehicle hydraulic brake systems.
In a typical vacuum assisted booster as shown in British Pat. No. 964,289 and as illustrated in FIG. 1 of the accompanying drawing, the vacuum booster input rod 11 has a control member 12 on the end thereof. The control member 12 slides in a bore 13 in a movable wall 14 and acts as a thrust member against a load transfer means 15 in the form of an elastomeric disc. The load transfer means 15 abuts against the piston head 16 of an output rod 17. The control member 12 is biased towards an "at-rest" position by a spring 18 acting on the input rod 11. In order to retain the control member 12 in its at-rest position a transverse key 19 is mounted in the movable wall 14 to interconnect with the control member 12. The key is arranged to permit forward displacement of the control member towards the elastic disc, but limits the return displacement of control member caused by the bias of the spring 18, and prevents the control member from being withdrawn from the movable wall.
The provision in the movable wall for the key 19 takes up a larger volume of space than is desirable.